Next-generation communication systems are configured to perform high-speed data transfers in order to provide good quality of service (QoS) to a user. The next-generation communication systems use orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) instead of conventional time division multiple access (TDMA) or code division multiple access (CDMA).
For example, OFDM or OFDMA may be applied to mobile WiMAX systems (IEEE 802.16e-2005), etc.
OFDM is a type of multiple carrier modulation method. According to OFDM, serial data are converted to parallel data and each of the parallel data is modulated into a plurality of subchannels having mutual orthogonality.
Intersymbol interference (ISI), which is originated from multi-path delay spread channel, is inevitable in a mobile communication system and is an important issue for the communication system design. In the system based on OFDM, the effect of ISI changes to the channel variations at frequency domain under the adequate system parameter setting. Therefore, only one-tap equalization is necessary for OFDM system, which is superior to comparing with CDMA and TDMA systems.
In the same manner as other mobile communication systems, the OFDM system uses an interleaving to prevent burst errors and channel coding to improve transmission quality when the signal encounters disturbances (noise, interference, multi-path propagation, etc.).
A receiver based on OFDM may select a hard decision method or a soft decision method in order to decode the channel encoded data.
For example, the hard decision method determines whether a data bit received in a specific section is ‘0’ or ‘1’ based on a predetermined standard (for example, the halfway point of a specific section), and the soft decision method divides a specific section into a plurality of equal intervals (that is, fixed quantization step sizes) and determines a data bit received in the specific section based on the divided intervals. For example, when a resolution is 3 bits, the soft decision method divides the specific section into eight intervals and determines the bits from a range of ‘0’ to ‘7’.
In the soft decision method, the number of bits used when the receiver of the communication system based on OFDM performs a quantization operation is related to the performance of the receiver. When the number of bits used in the quantization operation is relatively low, the efficiency of channel decoding decreases, whereas when the number of bits used in the quantization operation is relatively high, the complexity of the channel decoding and power consumption increase.
Therefore, the receiver needs to perform the quantization operation according to several conditions. That is, the receiver needs to variably determine the intervals (that is, a quantization step size) that are a standard of the soft decision method in order to reflect the change in characteristics of a log-likelihood ratio (LLR) of an input signal.